Concentrating on the task helps with nervousness quite a bit. It's when the airplane does something you didn't expect that you really get the jitters, and that's a result of not concentrating on your flying. Do the simple maneuvers over and over in order to build the confidence that you can control the airplane. It also helps to get up high and practice getting out of mistakes- inverted recovery, stall recovery, and nose dives. The basic issue is having the confidence to know that you are truly controlling the airplane and that it will do what you want it to do.